I bought a programmable timer for my HRV. It turns out the timer doesn't have battery, so when the power goes out the programming is lost. Since the power to the timer is coming from the HRV, how would I go about adding a battery backup to the timer. There are three wires going from the HRV to the timer. A coworker said the three wires would likely be power, signal, and ground - is that true? I measured the voltage across each pair of wires, when the timer is switched off (at the timer), and got these values: A-B: 4.53V A-C: 4.78V B-C: 0.25V. When the timer was switched on, the voltages were: A-B: 4.53V A-C: 4.88V B-C: 0.36V. At that time it was running the HRV fan on low speed. What sort of circuit would I need to ensure the following: